Graphene-Derived Supports for Hydroprocessing Catalysts - Industrial

Review pubs.acs.org/IECR

Graphene-Derived Supports for Hydroprocessing Catalysts Edward Furimsky* IMAF Group, 184 Marlborough Avenue, Ottawa, Ontario, Canada K1N 8G4 ABSTRACT: The graphene-derived solids such as doped graphene, graphene oxide (GO), and reduced graphene oxide (rGO) have been identified as supports for preparation of catalysts for various catalytic applications. Among those, hydroprocessing (HPR) of model compounds as well as feeds from conventional and nonconventional sources has also been evaluated. It was evident that HPR catalysts supported on graphene supports out-performed catalysts supported on other carbon solids. The stability of the former catalysts in the presence of water is of particular importance for the HPR of nonconventional feeds. Graphene solids as supports may play an important role during the development of HPR catalysts not requiring presulfiding. The activity of graphene alone was enhanced either by doping with various dopants or by functionalization. Once active metals were anchored on the surface, the activity of graphene-supported catalysts was increased significantly. Conventional metals such as Mo, Ni, Co, and Fe as well as noble metals such as Pt, Pd, Ru, and Rh have been evaluated as active metals.

1. INTRODUCTION Since discovery of graphene, a wide range of practical applications have been identified. For example, the potential of graphene material in biological applications has been reviewed by Sanchez et al.1 and Thompson et al.2 while various options in energy conversion processes have been investigated by Sahoo et al.3 as well as Wang and Gilbertson.4 The graphene-supported catalysts during the production of ammonia have been evaluated as well.5 In addition, other applications of graphene materials in catalysis may be anticipated. This is the first attempt to review studies on graphene materials and graphene-supported catalysts used in HPR reactions such as hydrogenation (HYD), hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrocracking (HCR), hydrodeoxygenation (HDO), and hydrodechlorination (HDCl). In view of the remarkable activity of the graphene-supported catalysts compared with the catalysts supported on other carbon supports,6,7 the HPR severity range in terms of temperature and H2 pressure may be widened to lower temperature compared with conventional HPR, i.e., 100 to 350 °C and 250 to 400 °C, respectively.8,9 However, even at less than 100 °C, graphene-derived materials out-performed other carbon solids (e.g., graphite, carbon black, activated carbon, and carbon fibers) used either alone or as the supports of Rhcatalyst used for the HDCl of 4-chlorophenol.10,11 In spite of a great potential of the graphene-supported HPR catalysts, relatively limited attention has been paid to their properties, preparation, and testing under HPR conditions. For example, an excellent collection of articles edited by Serp and Figueiredo12 in 2009 as well as the review published by Furimsky13 in 2008 mention the graphene-supported HPR catalysts only very briefly. However, since that time, a growing interest in these catalysts has been noted. © XXXX American Chemical Society

The primary objective of this review are the catalysts supported on graphene materials such as modified graphene, GO, and rGO used in various HPR applications. For comparison, the graphene materials alone were evaluated under similar conditions as well. Therefore, the properties and methods of preparation which are relevant to these applications are the focus of attention. Fuels and fuel components obtained over graphene-supported catalysts are the products of interest. The investigated feeds included various model compounds as well as the feeds derived from petroleum and unconventional liquids. Among the latter, the biomass derived model compounds and real biofeeds have been receiving most of the attention. In a few cases, nonfuel feeds and products were also included because the experimental conditions approached those encountered during HPR. Low severity (e.g., less than 100 °C) studies have also been noted because of a high activity of the catalyst. Some of these studies are also incorporated for comparison. In this case, they are referred to as HYD rather than HPR.

2. PROPERTIES OF GRAPHENE Pristine graphene is a single two-dimensional sheet of defectfree, polycyclic, hexagonally arranged, sp2-bonded aromatic carbon1,2,14. Few-layer graphene consists of a number of stacked sheets of pristine graphene (usually less than 10) and is a byproduct of the production of monolayer graphene. For applications in catalysis, irregularities, and/or defects in the structure of pristine graphene have to be created. This can be achieved by doping the graphene with heteroatoms such as Received: June 5, 2017 Revised: September 14, 2017 Accepted: September 18, 2017

A

DOI: 10.1021/acs.iecr.7b02318 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

approach 2,500 m2/g.23,24 According to Figure 1, graphene appears like a large aromatic molecule in the family of twodimensional poly aromatic hydrocarbons. Under certain H2 pressure, graphene is converted to graphane. In this process, carbon bonds configuration of sp2 in graphene is changed to sp3 in graphane.28 Graphene can be readily dispersed in a liquid phase from where it can be recovered either by filtration or centrifugation.24,25 With respect to catalyst preparation, this is an important property. Thus, while in suspension, nanosize particles of catalytically active metals can be anchored on graphene via d−π metal−graphene interaction. In addition, large surface area is favorable for a strong adsorption of reactant molecules. This facilitates catalytic action providing that such adsorption occurs in the proximity of active metals.24 A highly ordered pristine graphene is relatively inert and hydrophobic solid. However, functionalization of graphene can enhance hydrophilicity.26 For example, graphene becomes hydrophilic by introducing oxygen containing groups (e.g., carboxylic, hydroxyl, etheric).24 Then, catalyst preparation can be carried out under aqueous conditions. Moreover, Ocontaining groups are beneficial for anchoring active metals on graphene surface. Rather high thermal stability of the Ocontaining groups which are part of GO structure has been reported.29−32 Thus, according to Yang et al.,30 the temperature of at least 500 °C was necessary to notice thermal deoxygenation of GO. Once active metals are added to graphene, their interaction with the surface must ensure a long-term stability of catalyst under HPR conditions. First of all, the immobilization of nanoparticles of active metals must be maintained for a long period of time to prevent catalyst deactivation due to particle aggregation and leaching. In this regard, a necessary immobilization of active metal particles can be achieved by doping the graphene surface with heteroatoms.6,26 In the case of GO support, while the O-containing groups may facilitate deposition of nanoparticles of active metals on a graphene surface, the long-term stability of such groups under HPR conditions requires attention.13 The stability of carbon supports under HPR conditions is required to ensure resistance to methanation. For carbon nanofilaments and AC, such a reaction became evident over 450 and 550 °C, respectively.33 This confirmed that highly crystalline carbon nanosupports are much more resistant to methanation than relatively amorphous AC. Moreover, the remarkable activity of catalysts supported on either CNTs or graphene derived supports allows us to conduct HPR under less severe conditions. Nevertheless, even for extra heavy feeds, the HPR temperature rarely approached 450 °C.

nitrogen and sulfur. Figure 1shows the structure of pristine graphene and the graphene doped with nitrogen and/or sulfur.

Figure 1. Graphene and graphene-derived solids.

It is evident that the structure of pristine graphene was modified by creating irregularities due to replacement of some carbon atoms by either sulfur or nitrogen. Beneficial effect of other dopants (e.g., phosphorus and boron) was confirmed as well.15 Graphene oxide (GO) is a highly oxidized form of graphene produced by the oxidation of graphite, followed by the exfoliation of single monolayer sheets in solution.16 The structure of GO is shown in Figure 1. It is evident that the aromaticity of the pristine graphene can be partially reestablished by the reduction of GO to produce reduced graphene oxide (rGO) still containing some residual oxygen as well as the defects both on the basal plane and at the edges of sheets. If graphene are used as catalyst support, both the irregularities and residual O-containing groups play an important role during catalysts preparation and subsequent utilization during HPR and other reactions. The fuels related studies using non-HPR method over graphene based catalysts have also been noted.17 A source of proton is necessary, if GO and rGO are considered as supports for the preparation of bifunctional catalysts. For this purpose, Bronsted acidity can be introduced by sulfonation.16 Under suitable conditions (e.g., presence of water), a proton can be generated by the dissociation of sulfonyl groups, i.e.,

3. PREPARATION OF GRAPHENE Detailed accounts of the methods of graphene preparation have been given in several comprehensive reviews.6,24,25,34 Apparently, a gradual development of the methodology for the production of graphene began with micromechanical exfoliation of graphite performed by Fernandez-Moran in 1960.35 But, it took more than four decades before a single layer of 2D graphene was prepared by Novoselov et al.36,37 Theses authors cleaved graphite with an adhesive tape and transferred the separated layer onto a silicon wafer. Perhaps, the most developed methods to obtain higher yields of graphene-based nanomaterials are based on the graphite oxide exfoliation. Six decades ago, the preparation of graphite oxide was reported by Hummers and Offeman.38 In this case, powdered flake graphite

−SO3H → −SO3− + H+

Although less efficiently, proton can also be donated by carboxylic and hydroxyl groups present on the surface of GO and rGO. There is a wealth of information on various properties of graphene.18−26 In this regard, an introductory information available on Wikipedia is quite instructive.27 For the purpose of this review, the properties of graphene as support for catalyst preparation and catalyst performance during HPR are of primary interest. First of all, the morphology is the reason that graphene is among the solids with the largest surface area. Thus, theoretically, the surface area of pristine graphene may B


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Industrial & Engineering Chemistry Research was mixed with sodium nitrate and sulfuric acid. Potassium permanganate was added under vigorous agitation while the mixture was maintained in an ice bath. The temperature of the mixture was increased by removing the ice bath followed by the addition of water. The addition of hydrogen peroxide resulted in the reduction of residual permanganate and manganese dioxide to soluble manganese sulfate. The suspension was filtered to obtain graphitic oxide in the form of a yellowishbrown filter cake. The cake was washed with warm water and dispersed in water to remove remaining salt impurities using resinous anion and cation exchangers. Solid graphitic oxide was obtained by centrifugation followed by drying. The disruption of stacking in graphite oxide is necessary to separate the layers of GO. In this case, the layer exfoliation can be aided by ultrasonic, mechanical, chemical, and thermal methods. The exfoliation can be conducted in either an aqueous or organic environment.39 Various modifications of this method can be used and, in fact, have been adapted in some studies relevant for catalysis.40 Besides the top-down approach in preparation of graphene materials discussed above, bottom-up methods have also been receiving attention.26,41 It has been observed that single sheets of graphene could be prepared by the advanced method based on chemical vapor deposition (CVD) of graphene on metals and metal carbides.42,43 With this method, few or even single layer graphene can be prepared.44 Moreover, the deposition of a carbon atom and a heteroatom (e.g., nitrogen) can be carried out simultaneously, for example, using a hydrocarbon and ammonia as the source of carbon and nitrogen, respectively. Subsequently, an active metal can be added to the doped graphene using traditional methods such as impregnation45 According to the method published by Zhang et al.,46 the Fe/ graphene composite was prepared by dissolving glucose and FeCl3 in distilled water and evaporating the mixture at 80 °C for 24 h. The obtained solid was heated under N2 at 700 °C for 6 h. Apparently, a few layers of graphene could be obtained using this method. In this regard, Fe had a beneficial effect on the graphene formation. The brief account given above suggests that the preparation and pretreatment of graphene-derived material has been receiving much attention. Therefore, rather than to repeat this information in this review, the reader is referred to an extensive database in the literature, which includes several authoritative reviews.34−38,40−45

2. It must facilitate an activated adsorption of a reactant, enabling an interaction with active surface hydrogen. Apparently, it is not easy to fulfill these requirements for an ideal graphene structure such as is the case of pristine graphene. However, active sites on the surface of graphene can be readily generated by creating defects, by functionalizing (e.g., introducing O-containing and sulfonic groups), as well as by doping with heteroatoms such as nitrogen and sulfur as well as phosphorus, boron, and alkali metals).41−49 The study of Elias et al.28 showed that graphene can react with hydrogen atoms generated by cold hydrogen plasma. This observation was made during the experiments carried out below 160 K. In this process, graphene is converted to graphane which retains crystalline hexagonal lattice structure but the carbon bonds configuration of sp2 in graphene is changed to sp3 in graphane. This reaction is reversible; thus, the original structure of graphene can be restored by increasing temperature, e.g., by annealing in Ar at 450 °C for 20 h.28 However, the conditions under which the graphane was formed are remote to those encountered during HPR. Moreover, the relatively high temperature required to release hydrogen from graphane suggests that the activity and/or transferability of the surface hydrogen is low. Theoretical studies may contribute to the understanding of the structural features of graphene and corresponding hydrogenated products. For example, density functional theory was used to study the addition of atomic hydrogen to graphene.50 In other theoretical studies, the structural, electronic, and vibrational properties of hydrogenated graphene using firstprinciples calculations were investigated.51−53 It was indicated earlier that, during HPR, temperature may vary between 100 and 400 °C and the pressure of dihydrogen (H2) from atmospheric up to 10 MPa. It is believed that under such conditions, the HYD of a highly crystalline pristine graphene to graphane is rather slow. However, the rate of HYD may increase dramatically by introducing various dopants into the graphene structure. For example, during the HYD of anthracene, the surface hydrogen transfer to reactant over nitrogen doped rGO was significantly greater than that over unmodified GO.54 Based on the observations made by Elias et al.,28 one may prepare a modified graphene capable of adsorbing and activating hydrogen at a certain temperature but removing the active hydrogen from the surface by increasing temperature as is the case of typical HYD equilibrium, i.e., Graphene + H 2 ⇌ Graphane

4. CATALYTIC ACTIVITY OF GRAPHENES Carbon solids such as activated carbon (AC), carbon blacks (CB), carbon nano tubes (CNT), carbon nanofibers (CNF), and fullerenes are capable of adsorbing and activating hydrogen, i.e., converting dihydrogen to C−H entities.13 If a suitable reactant is in proximity, the active hydrogen can be transferred to initiate the HYD reaction. Such reactants include aromatics, olefin, heterorings compounds, etc. The potential of graphene in various catalytic applications was addressed in the review published by Albero and Garcia6 as well as Haag and Kung26 with limited attention being paid to HPR conditions. Two essential requirements must be fulfilled by graphene nanosheet to be catalytically active in HPR reactions, i.e., 1. It must convert dihydrogen to active surface hydrogen ,the availability of which is essential for HPR reactions to occur.

Graphane + Aromatics ⇌ Graphene + Naphthanes

Although the above conversions appear to be speculative, the transformations comprising multi-HYD equilibria have been observed during HPR, e.g., during HDS, HDN, and HDO of multiring heterocyclic compounds over carbon solids.13 Most recently, the HYD activity of graphene was confirmed by Liu et al.54 during the HYD of anthracene. The rGO used in this study was prepared from GO which was obtained from oxidized graphite according to the Hummers and Offeman38 method. In this case, GO was dispersed in deionized water, stirred, and then sonicated at room temperature. Subsequently, the solution was heated to 180 °C for 10 h in a Teflon-lined autoclave. The obtained rGO was separated by filtration, washed, and dried at 40 °C for 24 h. For doping with nitrogen, the rGO was treated with NH3 at 450, 650, 800, 900, 1000, and 1100 °C for 40 min. C


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Industrial & Engineering Chemistry Research The final products were denoted as A-rGO-Ts where “A” stands for ammonia and “T” temperature. Some structural parameters of these solids are shown in Table 1.54 In the Table 1. Absorption Capability of rGO (from the UV−vis Spectra) and CN/CC Ratio from FTIR [ref 54] FTIR

Graphitic N

rGO

Absorption, %

CN/CC

D/G

Raman values

rGO-450 “ 650 “ 800 “ 900 “ 1000 “ 1100

4 14 19 21 16 22

0.96 0.99 1.008 1.012 0.999 1.041

10.5 10.7 14.1 20.1 40.8 49.8.

2.66 2.54 2.49 2.51 2.18 1.83

Figure 2. Effect of rGO pretreatment on hydrogen transfer during HYD of anthracene [ref 54]. ■... rGO450; □...rGO650; ▼...rGO900; left facing tilted triangle...1100; ■...A-rGO450; ○...A-rGO450; ...ArGO800; ▼...A-rGO900; ◆...A-rGO1000; left facing tilted triangle...ArGO1100.

temperature range of 450 to 900 °C, the CN/CC ratio determined by FT-IR increased linearly from 0.96 to 1.012, respectively. This coincided with a mild decrease of D/G ratios. In this case, the Raman G parameter indicates the degree of graphitization while the D parameter the degree of irregularities in the A-rGO-T structure. A more pronounced decrease in the D/G ratio above 900 °C confirmed more extensive graphitization. A nearly linear increase between the amount of adsorbed anthracene and graphitic N suggests that the latter facilitated the π−π interactions of anthracene with the surface. The absorption capability in Table 154 was determined by UV− vis absorption spectroscopy of the A-rGO-T solids after being exposed to liquid anthracene. Shimoyama and Baba 55 demonstrated that doping with nitrogen and phosphorus enhanced significantly the absorption capability of graphene. In their study, thiophene was used as model compound. In the study of Liu et al.,54 the HYD of anthracene was carried out in a fixed bed reactor at 350 °C, 4 MPa H2, and LHSV of 2.5 h−1. It was evident that the A-rGO-T solids were much more active than the rGO-T as indicated by almost complete conversion of anthracene. The high activity was complemented by much higher hydrogen transfer capability of the A-rGO-T catalysts. Such hydrogen was determined from the amount of hydrogen which ended up in hydrogenated products. It was postulated that the synergy of graphitic N and the sp2 CC structure can enhance anthracene reactivity via π−π interactions while pyridinic N could facilitate hydrogen activation via dissociative adsorption. Thus, in the absence of graphitic nitrogen (e.g., in rGO-T solids), the hydrogen transfer from the surface to anthracene was much less evident. This is confirmed by experimental results shown in Figure 2.54

Hummers and Offeman.38 Table 257 shows that the Pd aided reduction of GO to rGO could be achieved at 200 °C during 2 Table 2. Composition of GO and rGOa [ref 57.]

a

Content

GO

rGO

Carbon, wt % Oxygen, wt % Atomic C/O ratio

75.1 15.7 4.8

89.6 6.5 13.8

Reduction at 200 °C for 2 h in H2 (5%) + Ar.

h in the flow of H2 (5%) + Ar. With respect to the structure of GO in Figure 1, the etheric oxygen in the basal plane would be removed most readily on the way toward rGO. Without Pd, little reduction of GO was observed. Therefore, Pd effectively activated dihydrogen to atomic hydrogens which migrated to the graphene surface and became available for the reduction of oxygen containing groups. Apparently, functionalization of graphene support is beneficial for catalyst preparation, particularly during impregnation, but the irregularities created by doping must be present to ensure active metals immobilization and the stable catalyst performance. It was indicated above that graphene alone can exhibit a wide range of activities in HPR reactions. The activity depends on the structure of the graphene surface and begins with nearly inert pristine graphene and attains a high activity of the graphene with highly irregular structure. The latter form is favorable for both hydrogen activation and activated adsorption of reactants. Hydrogen activation is significantly enhanced by the addition of active metals, particularly noble metals. The activated hydrogen produced on active metals spills over on the graphene surface which serves as a reservoir for active hydrogen. Therefore, if metals are present, the bare surface of graphene is more extensively covered with active hydrogen than that of the metal-free graphene surface. The methodology for preparation of the graphene-supported catalysts has been an active area of research as indicated by numerous studies referenced in the most recent review published by Navalon et al.7 It has been noted that various modifications of the method developed by Hummers and Offeman38,59 have been used most frequently. In a recent review, Zhu and Xu60 gave detailed accounts of the preparation of graphene-supported catalysts with emphasis on immobiliza-

5. GRAPHENE-SUPPORTED HPR CATALYSTS A single layer morphology ensuring a large surface area can facilitate an efficient dispersion of the particles of active metals on the graphene surface. To be suitable catalyst, an immobilization of active metals after their anchoring on the surface must be maintained. As was indicated above, the irregularities in graphene structure created by the addition of various dopants were observed to be of great importance.56 Also, the functionalizing of graphene, e.g., by introducing Ocontaining groups such as carboxylic, hydroxyl, and etheric as well as the sulfonic group is beneficial for anchoring active metals.57 However, the stability of such groups under HPR conditions is uncertain. For example, GO could be readily reduced to rGO as was shown in the study of Zheng et al.58 In this case, GO was prepared by the modified method of D


Review


MoS2) and the rGO alone exhibited very low activity. In spite of using an untypical model compound such as COS, these results clearly demonstrate the potential of graphene materials as supports for active catalysts used for HDS. The MoS2 catalyst supported on functionalized graphene sheets (GS) was prepared by sonicating the solution of ATTM mixed with functionalized GS for 30 min at room temperature.63 The obtained solid was vacuum-dried at 60 °C for 12 h and microwave radiated in a quartz reactor located in a microwave oven. In the final solid, the content of Mo as determined by ICP was about 10.5 wt %. In parallel, the catalyst was prepared by thermal treatment instead of microwave irradiation. In addition, the MoS2/AC was prepared for comparison. Figure 463 showed that during the HDS of COS

tion of nanosize active metal particles to prevent catalyst deactivation. It was evident that a large surface area of the doped graphene, GO, and rGO can prevent the loss of catalytic activity due to agglomeration of active metals particles and leaching. For functionalized graphene such as GO, metal particles can be anchored by dispersing in water and/or in a polar solvent together with an active metal precursor.46 In aqueous dispersion, noble metals can be reduced due to the difference in their redox potential and that of GO or rGO. Unwanted stacking of graphene sheets which may be observed should be minimized by selecting optimal preparation conditions as pointed out by Zhu and Xu.60 In this regard, a significant role may be played by the type of solvent as indicated in the study of Salavagione et al.61 In the present review, a brief account of catalyst preparation is given in relation to the activity of the catalysts being discussed. Both conventional metals (Co, Ni, Fe, Mo, and W) and noble metals (Pt, Pd, Ru, and Rh) supported on various forms of graphene have been evaluated for HPR applications. The feeds included both model compounds as well as those derived from biomass and petroleum. 5.1. Conventional Metals Supported on Graphene. In this section, the focus is on the catalysts supported on the doped graphene, GO, and rGO, comprising conventional metals. Theses metals have been essential active metals in conventional HPR catalysts, and they include Co, Ni, Mo, W, and Fe.8,9 The carbonyl sulfide (COS) is an uncommon reactant used to study catalyst performance. Under HPR conditions, COS is converted to H2S, CO, and CH4. In a systematic approach, the monolayer and few layers of MoS2 supported on rGO were compared during the HDS of COS.62 The catalyst preparation involved stirring GO with cetyltrimethylammonium bromide (CTAB) at 40 °C in water. This suspension was mixed with ammonium tetrathiomolybdate (ATTM) ultrasonicated in water as well as with hydrazine hydrate as reducing agent. The isolated precipitate was heated at 800 °C in N2 to obtain monolayer (M-MoS2/rGO) catalyst. Using a similar method but without CTAB, a few layers (F-MoS2/rGO) catalyst was obtained. The comparison of the catalyst was carried out between 140 and 300 °C in a continuous up-flow reactor in the flow of the H2 (20%) + N2 mixture. As Figure 362 shows, the M-MoS2/rGO catalyst was more active than the few layers FMoS2/rGO catalyst. Under the same condition, bulk MoS2 (B-

Figure 4. Effect of temperature and preparation method on catalyst activity [ref 63].

(500 ppm of COS in 10% H2 + N2 bal.), the microwave treatment produced much more active catalyst than the conventional thermal treatment. During the 10 h on stream test, the activity and stability of Mi-MoS2/GS at 260 °C were much higher than those of Th-MoS2/GS. Figure 563 showed that Mi-MoS2/GS catalyst was more active than MoS2/AC catalyst at all indicated temperatures. However, at 200 and 240 °C, bulk MoS2 was the most active. Yang et al.64 prepared graphene nanoribbons (GNR) by unzipping of CNTs and used them as the supports for MoS2. A single layer (SL-MoS2/GNR) and few layers (FL-MoS2/GNR) were prepared and compared with the pure multilayer MoS2 (ML-MoS2). The following catalyst activity order during the HDS of COS from 180 to 280 °C was established: SL-MoS2/

Figure 5. Effect of temperature and support type on catalyst activity [ref 63].

Figure 3. Effect of temperature on COS conversion [ref 62]. E


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Industrial & Engineering Chemistry Research GNRs > FL-MoS2/GNRs > ML-MoS2. The superior catalytic activity of SL-MoS2/GNRs was attributed to the high density of the active sites in single-layer MoS2. In another study on the HDS of COS conducted by Wang et al.,65 the monometallic Ni and Co as well as bimetallic Ni−Co catalysts supported on GS were prepared by the impregnation with metal acetates and subsequent reduction by dielectric barrier discharge (DBD) plasma in H2 atmosphere. The activity of these catalysts was evaluated in a continuous fixed bed reactor in the flow of H2 (10%) + N2 balance using COS as model reactant. The results of these evaluations are shown in Figure 6.65 The bimetallic catalysts exhibited a higher activity than monometallic catalysts. The catalyst prepared by the DBD reduction was more active than that reduced in the flow of H2.

Figure 7. Effect of catalyst type on HDS of DBT.

°C before the next experiment. Most of the original activity of the catalyst was recovered. In the study of Hajjar et al.,68 graphene was synthesized in a quartz tube reactor using the CVD method from either camphor or methane over nanosized copper catalysts under atmospheric pressure at temperatures of 900−1000 °C. Then, this compound was functionalized via oxidation in a mixture of concentrated nitric and sulfuric acids aided by ultrasonic treatment. After washing and drying, the solid support was concurrently impregnated with aqueous solutions of ammonium heptamolybdate and Co-nitrate. After separation from the solution, the solid was dried and calcined at 350 °C. The tests were conducted in a continuous fixed-bed reactor. Before tests, the catalysts were sulfided at 310 °C and 30 bar of H2 for 12 h using a mixture of 1 wt % DMDS in isomerate. The HDS runs were carried out at 300 °C and 1.5 MPa H2 using real feeds such as naphtha and diesel. The sulfur removal during these tests, recorded after 120 h on stream, is shown in Table 3.68 The conventional CoMo/Al2O3 catalyst was also used for comparison. The superiority of the CoMo catalyst supported on camphoric graphene was quite evident as is shown in Figure 8.68

Figure 6. Effect of temperature and catalyst type on COS conversion [ref 65].

Wang et al.66 compared the unmodified NiMo/TiO2 catalyst with the NiMo catalyst supported on a novel support such as graphene covered mesoporous TiO2. The latter was prepared from GO. In this case, the mixture of GO in distilled water and ethanol was ultrasonically treated for 1 h before the addition of TiO2. This suspension was transferred to a 40 mL Teflonsealed autoclave and maintained at 120 °C for 3 h. The resulting composite (TGC−x) was rinsed with deionized water and dried at 60 °C for 5 h. In the TGC−x support, x represents the mass ratio of rGO/TiO2. The final step included incipient impregnation of the support with nickel nitrate hexahydrate and ammonium heptamolybdate. The obtained powder was calcined at 500 °C for 2 h to obtain NiMo/TGC-x catalyst. After sulfiding in CS2 + H2 mixture, the catalyst was used for the HDS of DBT at 280 °C, 2 MPa, and LHSV of 4 h−1. For these tests, the mixture of DBT (1300 ppm of sulfur) in decalin was used. Figure 766 shows that the DBT conversion for unmodified NiMo/TiO2 and NiMo/TGC−0.5 was about 79 and almost 100%, respectively. Wei et al.67 prepared the catalyst which consisted of Co nanoparticles encapsulated in nitrogen doped graphene layers. This ensured an efficient dispersion of Co. Consequently, the agglomeration of the Co nanoparticles was prevented. Also, the losses of Co due to leaching during the HYD of quinolines (autoclave; 120 °C; 3 MPa of H2) in methanol solvent were minimized. The objective was to obtain tetrahydro products rather than complete HDN. Nevertheless, the catalyst exhibited a high activity and stability. To test recyclability, at the end of the experiment, the catalyst was separated from the reaction mixture by centrifugation, washed with ethanol, and dried at 70

Table 3. Sulfur Content (ppm) in Products from HDS of Naphtha (1350 ppm) and Diesel (13,000 ppm) [ref 68] Naphtha

Diesel

Catalysts

Product

% S remov.

Product

% S removed

CoMo/G Camphoric CoMo/G Methanic Conventional HPR

0 20 100

100 99 93

15 21 1937

99.9 99.8 85.1

In another study conducted by Hajjar et al.,69 the HDS activity of Co-MoS2/GO catalyst was compared with that of the CoMo/Al2O3 catalyst in a continuous fixed bed reactor at temperatures ranging from 250 to 315 °C, 1.5 MPa, and LHSV of 5 to 10 h−1. The former catalyst was prepared by a simultaneous exfoliation of graphite and MoS2 powder. The naphtha feed used for the study contained 1350 ppm of sulfur. At 315 °C, a complete sulfur removal was achieved over CoMoS2/GO catalyst compared with 100 ppm of sulfur still remaining after the test with Co−Mo/Al2O3 catalyst. F


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Table 4. Product Distribution from HDO of 4-Methyl Phenol in Absence and Presence of Water [ref 72] NiBPO NiBPOa NiBPO/ rGO NiBPO/ rGOa CoBPO CoBPOa CoBPO/ rGO CoBPO/ rGOa

4-MCHol

4-MCHon

3-MCHen

MCHan

TOL

DEOX

40.7 64.5 0.3

5.7 9.3 0.4

0.4 0.2 0.0

52.6 24.9 99.9

0.7 1.1 0.1

55.5 29.7 99.9

0

0.9

93.2

5.9

99.2

4.1 4.0 0.1

0.2 0.1 0.3

60.3 53.3 96.1

5.8 2.2 3.3

66.4 56.3 99.2

0

1.3

91.8

6.9

98.9

0 30.4 40.4 0.2 0

a

Water/phenol = 3/8; 4-MCHol = 4-methylcyclohexanol; 4-MCHon = 4-methylcyclohexanone; 3-MCHen = 3-methylcyclohexene; MCHan = methylcyclohexane; TOL - toluene; DEOX - deoxygenation degree.

Figure 8. Effect of graphene support type on HDS activity of CoMo/ G catalysts [ref 68].

Carbon materials, such as rGO, AC, glassy spherical carbon, and mesoporous carbon were used as supports for the Mo2C catalysts which were evaluated during the HDO of oleic acid to diesel hydrocarbons.72 The tests were conducted in a continuous fixed bed reactor (350 °C; 5.0 MPa of H2; LHSV of reactant varying from 2.0 to 8.0 h−1). In all respects, the Mo2C/rGO catalyst exhibited a superior activity compared with the Mo2C catalysts supported on other carbon supports. In addition, the most active Mo2C/rGO catalyst was compared with the conventional sulfided CoMo/Al2O3 catalyst during the HDO of soybean oil. Much higher stability of the former catalyst was indicated by only 16% conversion decrease compared with 42% decrease for CoMo/Al2O3 catalyst after 6 h on stream. Novel catalysts prepared by Wang et al.,71 consisting of the oxygenated form of the Ni- and Co-boride/phosphide phases, were used for the HDO of 4-methyl phenol in a batch reactor (0.1 g catalyst; 4.8 g phenol in 28.5 g of dodecane; 225 °C; 4 MPa H2; 1 h). The parallel experiments were carried out in the presence of water using a molar ratio of water phenol of 3:8. Both unsupported and rGO-supported forms of catalysts were compared under identical conditions without and with water being present. In every case, almost complete conversion of 4methyl phenol was achieved. However, as the results in Table 4 show,71 water had an adverse effect on the product distribution over the unsupported catalysts while the effect over the rGOsupported catalysts was rather minor. The study of Yuan et al.72 might be the only of its kind in which the Ni-graphene nanocomposite was used as catalyst for the reduction of viscosity of extra heavy feed. The experiments lasting 24 h were conducted in an autoclave at 280 °C using 50 g of heavy feed and 0.5 g of the nanocomposite containing 2% rGO. More than 80% viscosity reduction was achieved in the presence of 3 wt % of tetraline (on the weight of feed). The preparation of catalyst involved mixing 2.00 g of NiCl2·6H2O and 0.24 g of NaBH4 with 50 mL of ethylene glycol under magnetic stirring in a 100 mL beaker. Subsequently, the mixture was sonicated together with rGO for 30 min and heated between 120 and 180 °C. After cooling, the solid was separated by centrifuging, washed with water, and vacuumdried. Xing et al.73 used the Fe/graphene nanocomposite prepared by the method of Zhang et al.46 for the viscosity reduction of a Chinese heavy oil. In a typical experiment, 100 g of heavy oil and 1 g of catalyst were mixed in a 200 mL autoclave and kept

at 200 °C for 24 h. For Fe/graphene catalyst, viscosity reduction of 64% was achieved compared with 38% reduction in the absence of catalyst. Significantly higher activity of MoS2//rGO compared with the bulk MoS2 (Figure 3)62 as well as that of CoMoS2/G compared with CoMoS2/Al2O3 (Figure 8)68 can be attributed to a much more efficient dispersion of MoS2 on the graphene derived supports. This facilitates a higher concentration of active sites. In addition, electronic effects of the graphene surface may modify the geometry of active sites, making them more suitable for hydrogen activation. In this case, the −C− Mo−S−H and −C−Mo−H entities may be involved.74 So far, no evidence has been provided to support the involvement of the Co−Mo−S phase which has been confirmed in oxidic supports.75 Then, for CoMoS2 supported on graphene derived supports, the presence of −C−Co−S sites cannot be ruled out. Thus, a high activity of the Co/rGO catalyst has been reported.67 Based on this hypothesis, a parallel involvement of different active sites in the CoMo catalysts supported on graphene derived supports may be anticipated. 5.2. Noble Metal-Containing Catalysts. The hydrophobicity of graphene support ensures the stability of noble metals catalysts either in the presence of large quantities of water or directly in an aqueous phase. Such conditions may be encountered during the HPR of biomass derived feeds and FT syncrude. Attempts have been made to test noble metals containing catalysts supported on graphene materials under aqueous conditions. It has become evident that the immobilization of active metal particles on the supports surface is crucial for the stability of noble metals containing catalysts. Otherwise, catalyst deactivation due to the agglomeration of active metal particles as well as particles loss due to leaching could not be avoided. 5.2.1. Catalyst Testing under Aqueous Conditions. Because of its hydrophobicity, graphene has a potential to be a suitable support for HPR catalysts under aqueous conditions as confirmed by an extensive database of HPR catalysts supported on other carbon supports (e.g., AC, CBs, CNTs, CNFs, etc.).13,76 It was observed that in some respects, carbonsupported catalysts outperformed the HPR catalysts supported on traditional supports (e.g., γ-Al2O3 and SiO2). In the study of Ibrahim et al.,77 Pd nanoparticles were incorporated into a nanocomposite consisting of Ce-based metal−organic framework crystals and partially reduced GO G


Review

Industrial & Engineering Chemistry Research Table 5. Composition of Supports and Catalysts and Activities of Catalysts [ref 78]a Composition, wt % GO rGO rNrGO−150 NrGO−300 Pd/rGO Pd/GO Pd/NrGO−150 Pd/NrGO−300 a

C

O

62.5 80.8 84.4 79.9 78.1 80.0 80.6 75.9

37.5 19.2 13.2 12.6 17.7 15.5 12.3 12.2

Conversion, %

N

Pd

Selectivity, A

B

87.3 86.4 93.4 95.9

12.7 13.6 6.6 4.1

2.4 7.5 4.5 4.6 4.5 4.6

2.5 7.3

9.0 8.8 51.5 82.5

A Hydrocinnamal aldehyde. B Hydrocinnamal alcohol.

nanosheets. The catalyst was used for the HDO of vanillin in a batch reactor at 100 °C and 1 MPa of H2. For the experiments, 2 mmol of vanillin and 50 mg of catalyst both dispersed in water were used. Under these conditions, a high HDO activity of the catalyst was noted. After separating from the reaction mixture, the spent catalyst was washed with DMF and ethanol, and oven-dried at 100 °C before being reused for another cycle. For the spent-washed and dried catalyst, little loss of activity was noted during the subsequent test. Nie et al.78 used two rGO supports for the preparation of Pd/rGO catalysts. In this case, GO was dispersed in deionized water together with urea. After sonication for 3 h, the mixture was kept in an autoclave at 180 °C for 10 h. This produced nitrogen reduced graphene oxide (NrGO) support. Two supports, one with a urea/graphene oxide ratio of 300 and 150, i.e., NrGO−300 and NrGO−150, respectively, were prepared. A similar method without urea produced the rGO support. Both supports were used for the preparation of Pd catalysts. In this case, PdCl2 was added to the suspension of support in water and stirred for 0.5 h. Subsequently, the suspension was treated in an autoclave at 40 °C for 8 h under 2 MPa H2. The composition of catalysts and supports together with the activities of catalysts are shown in Table 5.78 It is evident that the modification of graphene support with nitrogen from urea had a significant effect on catalyst activity The activities were determined in an autoclave using 2 mmol of reactant, 0.01 mol % Pd as catalyst in 2.5 mL of H2O at 70 °C, 0.5 h, and 2 MPa H2. In this case, the model reactions such as the HYD of the CC bond in cinnamaldehyde and the HYD of phenol to cyclohexanone were used. A high selectivity of the catalysts for these reactions under rather mild conditions should be noted. Wang et al.79 developed the methodology for preparation of Ru/rGO which was subsequently sulfonated to introduce sulfonic (−SO3H) groups to the rGO surface. Both catalysts were used for the conversion of levulinic acid (LA) in an aqueous solution at 50 °C and 2 MPa of H2 in an autoclave. Almost complete conversion of LA was achieved during a 40 min test over both Ru/rGO and the sulfonated (Ru/rGO-s) catalyst. However, as Figure 979 shows, the selectivity to the formation of γ-valerolactone over the latter catalyst was significantly greater. This was attributed to the bifunctional nature of the Ru/rGO-s catalyst facilitated by the sulfonic group. Thus, the hydrogenated intermediate such as 4hydroxyvaleric acid formed during the first step of the overall conversion of LA was converted via dehydration to γvalerolactone over Ru/rGO-s at a much greater rate compared with that over Ru/r-GO. In this case, proton aided dehydration was involved. As indicated earlier, the sulfonic group was the main source of protons.

Figure 9. Effect of sulfonation of rGO support on activity of Ru/rGO catalysts [ref 79].

5.2.2. Testing without Water in Feed. During these tests, water was not present in the feed, initially. However, in the case of the O-containing reactants, water was formed as the reaction byproduct. It should be noted that in a continuous system water is formed and continuously removed from reactor, contrary to batch systems where water accumulates in the reactor. Therefore, a significant difference in the effect of water on catalysts performance between continuous and batch systems should be anticipated. In the study of Upare et al.,80 the preparation of Ru/GO catalyst involved soaking GO in an aqueous solution of RuCl3. The resultant mixture was sonicated for 1 h and aged for 12 h at room temperature. The powder obtained after removing the water in a rotary evaporator was dried at 120 °C and reduced in the flow of H2 (10%) balance N2 at 450 °C for 2 h. The Ru/AC catalyst was prepared in similar manners. The catalysts were used for the HYD of levulinic acid (LA) at 265 °C and H2 pressure varying from 1 to 2.5 MPa. The experiments were conducted in a down-flow continuous fixed bed reactor. For the experiments, 10 wt % of LA in 1,4-dioxane solvent, were used. The HYD of LA over Ru/GO yielded 54 and 41% of cyclic ethers such as MTHF and THF as well as γ-valerolactone, respectively, while the latter was the only product over Ru/AC catalyst. The Pt/rGO was compared with the Pt catalysts supported on conventional supports such as Al2O3 and SiO2 during the HYD of 5-hydroxymethyl furfural (HMF) to 2,5-dimethyl furan H


Review

Industrial & Engineering Chemistry Research (DMF), a high octane component for blending with gasoline.81 The experiments were conducted in an autoclave. In this case, HMF mixed with butanol and catalyst in the reactor before purging with an inert gas and pressurizing with H2 to 3 MPa. The results of these tests are summarized in Table 6.6 These

reactant and n-decane. To study the reusability, the Ru/rGO catalyst was separated from the reaction mixture by centrifugation, and washed with acetone and toluene before the next reaction run. Table 782 shows that almost a complete Table 7. Effect of Carbon Support on Activity of Ru Catalysts for HYD of Toluene and Benzene [ref 82]a

Table 6. Activity and Selectivity of Catalysts for HYD of HMF [ref 6]a

T i m e, min

Yield, % of carbon

Catalyst rGO PtNPs Pt/rGO Pt/AC Pt/SiO2 Pt/Al2O3

15

Pt cont., wt %

Pt partic. size, nm

Conv. %

DMF

MFF

BHMF

MFM

4.9 4.5 3.9 4.1

0 4.9 2.8 3.3 4.6 5.7

10.5 35.6 100 67.4 52.1 45.2

0 19.9 73.2 32.6 18.2 16.4

2.9 1.0 1.6 2.5 0 0.9

1.4 0.9 0.8 2.8 0 0

1.0 1.1 0.6 0 2.2 1.0

Toluene/Ru = 8.8 × 103 Ru/rGO1 fresh 39.1 after 6 cycles 38.5 Ru/MCF-C 33.6 Toluene/Ru = 5.3 × 104 Ru/rGO 11.6 Ru/CAC 5.8 Ru/MCF-C 4.6 Benzene/Ru = 5.3 × 104 Ru/rGO 14.5 Ru/MCF-C 6.1

a

DMF = 2,5-dimethylfuran; MFF = 5-methylfurfural; BHMF = 2,5bishydroxymethylfuran; MFM = 5-methyl-2-furanmethanol.

results were obtained under the following reaction conditions: 2.0 mmol of HMF; catalyst equivalent of 0.02 mmol Pt; 12 mL of 1-butanol, H2 of 3.0 MPa; stirring rate, 1000 rpm; 120 °C; 2.0 h. The best performance of the Pt/rGO catalyst compared with other catalysts is quite evident. Moreover, a high yield of DMF should be noted. This can be attributed to the highest dispersion of Pt on rGO support. In the study of Yao et al.,82 the highest activity during the HYD of benzene and toluene was exhibited by Ru/rGO catalyst compared with Ru/MCF-C and Ru/CAC catalysts. The difference in activity in favor of the Ru/rGO catalyst became much more evident at the higher toluene/Ru ratio, i.e., when the availability of active sites became more critical. The support denoted as MCF-C was prepared using a hard-templating method from siliceous meso-cellular foam while CAC was an amorphous carbon. The higher HYD activity of the Ru/rGO catalysts than that of the Ru/MCF-C catalyst coincided with much stronger adsorption of toluene on the former as indicated in Figure 10.82 Thus, the maximum of the toluene desorption peak over Ru/rGO was about 80 °C higher than that of the Ru/MCF-C. Prior to the activity tests, catalysts were activated in NaBH4 solution at 100 °C for 20 h and used in an autoclave at 130 °C and H2 pressure of 4 MPa using the mixture of

a

30

45

60

88.8 86.5 41.8

99.8 98.2 44.7

100 100 45.1

18.9 7.7 9.6

25.5 9.0 15.0

34.3 12.6 21.0

30.2 10.0

44.7 14.2

61.3 19.8

Conditions: autoclave, 130 °C, 4 MPa H2.

recovery of catalyst activity was achieved after six repeated runs. In another study, a high activity for the HYD of benzene was also observed using the Ru catalyst supported on an ionic liquid stabilized graphene.83 Moreover, this catalyst could be reused without a detectable loss of activity. The functionalized GO prepared by thermal exfoliation of graphite oxide at 200 °C was used as the support for Ru nanoparticles of 2.3 nm size.84 For comparison, the Rusupported on carbon nanotubes (CNT) was also tested. The catalyst preparation involved the incipient wetness impregnation followed by heat treatment at 700 °C in N2 flow before being used for the HYD of several reactants including benzene. The experiments were carried out in an autoclave at 70 °C and 1 MPa of H2. Under these conditions, the Ru/graphene exhibited a higher activity for the HYD of benzene than Ru/ CNT catalyst. Of course, those were untypical HPR conditions. However, this is one of the few studies in which graphene support was compared to CNT support. The catalyst comprising a multifunctional Pd/Zr-based metal−organic framework supported on sulfonated GO exhibited Bronsted acidity during the conversion of fructose to 5-hydroxymethylfurfural followed by Pd aided HYD and hydrogenolysis of the latter to 2,5-dimethyl furan which is a known octane buster additive to gasoline.85 More than 70% conversion to 2,5-dimethyl furan was achieved at 160 °C and 1 MPa of H2 during a 3 h experiment in a batch reactor. A high recyclability of the catalyst was noted by little loss in activity after five repeated cycles. Table 8 summarizes the studies on development and testing of the catalysts supported on graphene based supports to be used in various HPR applications. The first glance look at the results involving conventional metal catalysts indicates less severe conditions compared with those encountered over commercial HPR catalysts. This can be attributed to the beneficial effect of the graphene-derived supports compared with conventional supports. The HPR severity was further decreased by combining graphene supports with noble metals. For these catalysts, a good performance was observed under aqueous conditions. These results suggest that the potential of

Figure 10. Effect of catalyst type on adsorption of toluene [ref 82]. I


Review


to a π−d interaction between the graphitic surface and Pt.87 For an amorphous carbon black, such an interaction was not observed. Wang et al.88 obtained functionalized GO by explosive expansion of graphite oxide. The GO was treated at room temperature under air, N2, and H2 to obtain GOa, GOn, and GOh, respectively. These GOs were used as the support for preparation of Ru catalysts by incipient wetness impregnation. After drying at 110 °C and reducing at 400 °C in H2, the catalysts were further heat treated at 700 °C. Apparently, the heat treatment at 700 °C under N2 made Ru nanoparticles electron rich. These catalysts were compared with the Ru catalysts supported on CNTs prepared by a similar impregnation method. The testing of these catalysts was conducted in an autoclave at 373 K and 1 MPa H2 using cinnamaldehyde, benzene, and p-chloronitrobenzene (p-CNB). The results in Table 1088 show that during the HYD of

Table 8. Catalysts Supported on Graphene Derived Supports Catalysts

Reactant/ Feed

Conventional metals COS MoS2/rGO MoS2/GSa

COS

Ni,Co & Ni-Co/GS

COS

NiMo/rGO-TiO2 Co/N-doped G CoMo/G function.

DBT Quinoline Naphtha; diesel Oleic acid Cresol Heavy feed

Mo2C/rGO Ni, CoBPO/rGO Ni/rGO, Fe/rGO Noble metals Pd/rGO Pd/rGO

Phenol

Ru/rGO

Levulinic acid

Ru/rGO

Levulinic acid

Ru/rGO

Benzene, toluene Benzene HMHb Fructose

Ru/rGO Pt/rGO Pd−Zr/GO a

Vanilin

Conditions 140−300 °C; H2 + N2; cont. 200−300 °C; H2+N2; cont. 140−300 °C; H2+N2; cont. 280 °C; 2 MPa; cont. 120 °C; 3 MPa; batch 300 °C; 1.5 MPa; cont.

Reference 62 63 65 66 67 68

350 °C; 5 MPa; cont. 225 °C; 4 MPa; batch 280 °C; batch

70 71 72, 73

100 °C; 1 MPa, H2O; batch 100 °C; 2 MPa, H2O; batch 50 °C; 2 MPa; H2O; batch 265 °C; 1−2.5 MPa; cont. 100 °C; 4 MPa; batch

77

70 °C; 1 MPa; batch 120 °C; 3 MPa; batch 150 °C; 1 MPa; batch

Table 10. Activity and Selectivity during Cinnamaldehyde Conversiona [ref 87]

78

Selectivity, %

79 −1

Catalyst

80

TOF, h

Ru/GOa Ru/GOn Ru/GOh Ru/GOa-ht Ru/CNTa Ru/CNTa-ht

82 84 81 85

Graphene sheets. bHydroxymethyl furfural.

119.4 115.4 122.6 104.6 52.4 45.2

COL

HCAL

HCOL

Others

30 30 31 30 32 39

55 52 52 55 52 49

8 8 8 7 8 8

7 10 9 8 8 4

a

TOF and selectivity calculated at 40% conversion. COL = cinnamyl alcohol; HCAL = hydrocinnamaldehyde; HCOL = hydrocinnamyl alcohol; Others = acetals and semiacetals.

the HPR catalysts supported on graphene-derived supports has not yet been fully realized. 5.3. Comparison of rGO- with CNT-Supported Catalysts. Among other carbon nanomaterials, i.e., carbon nanotubes (CNTs), carbon nanofibers, carbon nanohorns, and fullerenes, the CNTs have been attracting most of the attention. Few studies involved a direct comparison of the CNTsupported catalysts with those supported on the graphenederived supports, usually under low severity conditions. The results obtained during the conversion of cinnamaldehyde over Pt catalysts, such as Pt(2.9%)/rGO, Pt(4%)/CNT, and Pt(4%)/AC, are shown in Table 9.86 With respect to the conversion and selectivity to cinnamon alcohol, the best performance was exhibited by Pt/rGO. The activity expressed as moles of converted reactant per mole of Pt confirmed superior performance of this catalyst as well. Of particular significance is the relatively low activity of the Pt/AC catalyst in spite of the rather high surface area. This clearly confirmed that electronic factors play a dominant role in the catalysis over rGO- and CNT-supported catalysts. Thus, the electronic modification of Pt particles by these supports was attributed

cinnamaldehyde, the activity of the GO-supported catalysts was much higher than that of the CNT-supported catalysts although the difference in selectivity was less evident. The heat treatment had an adverse effect on TOFs of former catalysts. A higher activity of the GO-supported catalysts was also observed during the HYD of benzene; however, the CNTsupported catalysts were more active during the HYD of pCNB (Table 11).88

6. FUTURE PERSPECTIVES It has been demonstrated in several studies that catalysts supported on graphene materials (e.g., modified graphene, GO, and rGO) out-performed other carbons as catalyst supports during HYD and HPR of various feeds. In spite of remarkable activity, relatively limited attention has been paid to these catalysts. Their stability under aqueous conditions is of particular importance for the HPR of nonconventional feeds such as biofeeds, CDLs, FTS syncrude, and light tight oils. Because of a higher activity and stability of the graphene-

Table 9. Effect of Support Type on Cinnamaldehyde Conversion (%) and Selectivity (%) [ref 86]a Surf. Area

Selectivity

Catalyst

m2/g

Conver.

COL

HCAL

HCOL

Others

Activityb

Pt/AC Pt/CNT Pt/rGO

1029 158 276

84 87 90

30.9 48.3 69.6

27.0 20.8 9.2

35.2 26.2 17.6

6.8 4.7 3.6

311 322 456

a

COL = cinnamyl alcohol; HCAL = hydrocinnamaldehyde; HCOL = hydrocinnamyl alcohol; Others = acetals and semiacetals. cinnamaldehyde/mol Ptxh. J

b

Mol


Review


(8) Furimsky, E. Properties of tight oils and selection of catalysts for hydroprocessing. Energy Fuels 2015, 29, 2043−2058. (9) Furimsky, E. Hydroprocessing challenges in biofuels production. Catal. Today 2013, 217, 13−56. (10) Kalmykov, P. A.; Arbuzov, A. A.; Magdalinova, N. A.; Tarasov, B. P.; Klyuev, M. V. Palladium-containing graphene-like materials: Preparation and application as hydrogenation catalysts. Pet. Chem. 2016, 56, 503−509. (11) Klyuev, M. V.; Magdalinova, N. A.; Kalmykov, P. A. Metalcontaining graphene-like materials: Synthesis and use in hydrogenation. Pet. Chem. 2016, 56, 1093−1106. (12) Serp, P., Figueiredo, J. L., Eds.; Carbon materials for catalysis; John Wiley & Sons Inc. Publ.: Hoboken New Jersey, USA, 2009. (13) Furimsky, E. Carbons and carbon supported catalysts in hydroprocessing; RSC Publishing: Cambridge, UK, 2008. (14) Huang, Z.; Zhou, H.; Yang, W.; Fu, C.; Chen, L.; Kuang, Y. Three-dimensional hierarchical porous nitrogen and sulfur-codoped graphene nanosheets for oxygen reduction in both alkaline and acidic media. ChemCatChem 2017, 9, 987−996. (15) Shimoyama, I.; Baba, Y. Thiophene adsorption on phosphorusand nitrogen-doped graphites: Control of desulfurization properties of carbon materials by heteroatom doping. Carbon 2016, 98, 115−125. (16) Gómez-Martínez, M.; Baeza, A.; Alonso, D. A. Pinacol rearrangement and direct nucleophilic substitution of allylic alcohols promoted by graphene oxide and graphene oxide CO2H. ChemCatChem 2017, 9, 1032−1039. (17) Marso, T. M. M.; Kalpage, C. S.; Udugala-Ganehenege, M. Y. Metal modified graphene oxide composite catalyst for the production of biodiesel via pre-esterification of Calophyllum inophyllum oil. Fuel 2017, 199, 47−64. (18) Geim, A. K. Random walk to graphene (Nobel lecture). Angew. Chem., Int. Ed. 2011, 50, 6966−6985. (19) Novoselov, K. S.; Castro Neto, A. H. Two-dimensional crystalsbased heterostructures: materials with tailored properties. Phys. Scr. 2012, T146, 014006. (20) Geim, A. K. Graphene: Status and Prospects. Science 2009, 324, 1530−1534. (21) Geim, A. K.; Novoselov, K. S. The rise of graphene. Nat. Mater. 2007, 6, 183−191. (22) Geim, A. K.; MacDonald, A. H. Graphene: exploring carbon flatland. Phys. Today 2007, 60, 35−41. (23) Neto, A. H. C.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. The electronic properties of graphene. Rev. Mod. Phys. 2009, 81, 109−162. (24) Soldano, C.; Mahmood, A.; Dujardin, E. Production, properties and potential of graphene. Carbon 2010, 48, 2127−2150. (25) Radovic, L. R.; Mora-Vilches, C.; Salgado-Casanova, A. J. A. An assessment is offered regarding the progress made, and the remaining challenges, in the field of carbo-catalysis. Chin. J. Catal. 2014, 35, 792− 797. (26) Haag, D. R.; Kung, H. H. Metal free graphene based catalysts: A Review. Top. Catal. 2014, 57, 762−773. (27) https://en.wikipedia.org/wiki/Graphene. (28) Elias, D. C.; Nair, R. R.; Mohiuddin, T. M. G.; Morozov, S. V.; Blake, P.; Halsall, M. P.; Ferrari, A. C.; Boukhvalov, D. W.; Katsnelson, M. I.; Geim, A. K.; Novoselov, K. S. Control of graphene’s properties by reversible hydrogenation: Evidence for graphane. Science 2009, 323, 610−613. (29) Gao, X.; Jang, J.; Nagase, S. Hydrazine and thermal reduction of graphene oxide: Reaction mechanisms, product structures, and reaction design. J. Phys. Chem. C 2010, 114, 832−842. (30) Yang, D.; Velamakanni, A.; Bozoklu, G.; Park, S.; Stoller, M.; Piner, R. D.; Stankovich, S.; Jung, I.; Field, D. A.; Ventrice, C. A., Jr.; Ruoff, R. S. Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy. Carbon 2009, 47, 145−151. (31) Tung, V. C.; Allen, M. J.; Yang, Y.; Kaner, R. B. Highthroughput solution processing of large-scale graphene. Nat. Nanotechnol. 2009, 4, 25−28.

Table 11. Catalysts Performance during Conversion of Benzene and p-CNB [ref 87] Benzene Ru/GOa Ru/GOa-ht Ru/CNTa Ru/CNTa-ht Ru/GOa p-CNB Ru/GOa-ht Ru/CNTa Ru/CNTa-ht

Conversion, %a

TOF, h−1b

74.5 23.5 51.2 24.7 5.3

1302 411 622 375 93

9.8 14.3 32.5

172 174 494

a The conversion is recorded at half an hour for all data. bTOF (mol reactant/mol Ru × h).

supported catalysts, a desirable conversion of model compounds and real feeds could be achieved under less severe conditions. In this regard, these catalysts have a high potential to replace conventional HPR catalysts during the search to develop novel HPR catalysts not requiring presulfiding. For the graphene-supported catalysts, additional testing is needed that very positive results obtained in batch reactors are also confirmed in continuous catalytic systems. With respect to long-term performance, recyclability of these catalysts requires attention particularly for the catalysts containing noble metals. Apparently, preparation of the supports via a top down approach, i.e., starting with graphite through graphene, GO toward rGO, can be further optimized to make the catalysts economically attractive. It is believed that in terms of the availability of a wide range of precursors, the potential of the bottom-up approach has not been fully explored compared with the graphene-supported catalysts used in non-HYD and nonHPR applications.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Edward Furimsky: 0000-0003-1150-9067 Notes

The author declares no competing financial interest.

■

REFERENCES

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